This invention relates to a motor driving device.
Conventionally, an image forming apparatus, which forms an image on a recording sheet by ejecting drops of ink from nozzles of a recording head, has been known. In this type of image forming apparatus, when an image forming process is performed, a carriage, carrying a recording head, is moved by a motor in a main scanning direction so as to convey the recording head in the main scanning direction, the recording head is simultaneously driven so as to eject drops of ink on a recording sheet, facing the recording head, and, as a result, an image is formed on the recording sheet.
Specifically, in this type of image forming apparatus, based on an output of an encoder, which detects the position of the carriage, the speed of the carriage in an image formation area is controlled at a constant speed by a feedback control method. Generally, in order to move the carriage to a home position, the motor control method is changed and the carriage is moved at a minimal speed.
In an inkjet image forming apparatus, a cap, which covers the nozzle surface of the recording head is generally disposed in the home position in order to inhibit the nozzle surface of the recording head from being dried. For a cap mechanism, a type of cap mechanism wherein the cap is conveyed to the nozzle surface of the recording head when the cap mechanism receives the pressing force from the carriage in correspondence with the movement of the carriage, is widely adopted. In an image forming apparatus configured as above, if the carriage is moved to the home position at a high speed, the cap is likely to strongly contact with the nozzle surface of the recording head and damage the nozzle surface. Therefore, in a conventional technique, the carriage is moved at a minimal speed when the carriage is moved to the home position.
Specifically, as a method for moving a carriage at a minimal speed, a method is known wherein a motor is driven such that the amount of current inputted into the motor is temporarily decreased to an initial value every time the carriage is moved forward for a predetermined distance, and then the amount of current is gradually increased from the initial value, so that the carriage is moved at a minimal speed. In this method, when a pulse signal is inputted from an encoder, for example, a process is performed wherein the carriage is determined to be moved forward, and the amount of the current is changed back to the initial value.
In a case wherein a cap mechanism is also operated by the driving force of a motor that drives a carriage, the load applied to the motor fluctuates when the carriage is in the vicinity of a position wherein the pressing force, generated in correspondence with the movement of the carriage, is applied to the cap mechanism. Specifically, the load increases while static frictional force acts on the cap mechanism, and decreases from the peak when the movement of the cap mechanism is initiated. Generally, a maintenance mechanism, which draws residual ink adhered to nozzles, is disposed below a cap. For the driving source of the maintenance mechanism, a motor for driving conveyance rollers is often used. In this type of image forming apparatus, a gear is switched by the pressing force, generated in correspondence with the movement of the carriage, and thereby the maintenance mechanism is driven. For example, a switching mechanism is disposed so as to be slidable with respect to a guide, which restricts the moving direction of the carriage. The pressing force, generated in correspondence with the movement of the carriage, is applied to the switching mechanism so as to drive the switching mechanism, and consequently to switch the gear connected to the motor. In such case, the load suddenly decreases from the peak, when the carriage is in the vicinity of a position wherein the pressing force, generated in correspondence with the movement of the carriage, is applied to the switching mechanism as much as to overcome the maximum frictional force of the switching mechanism and the movement of the switching mechanism is initiated. Moreover, the amount of the load fluctuation changes as the cap mechanism deteriorates with age. There has been a problem in a conventional apparatus that load fluctuation cannot be appropriately handled when the load fluctuation becomes large due to deterioration with age and the like of the cap mechanism and the above-described switching mechanism, and therefore the carriage sometimes cannot be appropriately moved at a minimal speed to the capping position.
Moreover, in a general image forming apparatus, a carriage is moved along a guide shaft. When the carriage is moved at a minimal speed along the guide shaft, carriage is sometimes held up with the guide shaft and a load fluctuation is caused. However, in a conventional apparatus, if a large load fluctuation is caused in such situation, the load fluctuation cannot be appropriately handled, and, in some cases, the carriage cannot be suitably moved at a minimal speed.